172 RESPONSE IN THE LIVING AND NON-LIVING 



The length of the tube is so adjusted that the disc is at 

 the distance of most distinct vision from the end of the 

 tube applied to the eye. The blackened disc is turned 

 towards a source of strong light, and a short exposure 

 is given by the release of a photographic shutter inter- 

 posed between the disc and the eye. On closing the 

 eye, immediately after a short exposure, it will at first 

 be found that there is hardly any well-defined visual 

 sensation ; after a short time, however, the writing on 



FiG. 110. RESPONSE-CURVES or THE SENSITIVE SILVER CELL 



Showing greater persistence of after-effect when the stimulus is strong, 

 (a) Short exposure of 2" to light of intensity 1 ; (b) short exposure of 2" to 

 light nine times as strong. 



the blackened disc begins to appear in luminous 

 characters, attains a maximum intensity, and then fades 

 away. In this case the stimulus is of short duration, 

 the light being cut off before the maximum effect is 

 attained. The after-effect here is positive, there being 

 no reversal or interval of darkness between the direct 

 image and the after-image, the one being merely the 

 continuation of the other. But we shall see, if light is 

 cut off after a maximum effect is attained by long 



